Immune evasion of Borrelia burgdorferi by acquisition of human complement regulators FHL-1/reconectin and Factor H

To understand immune evasion mechanisms of Borrelia burgdorferi we compared serum-resistant B. afzelii and serum-sensitive B. garinii isolates for the ir capacity to acquire human complement regulators. Here we demonstrate tha t the two borrelial genospecies show different binding of the two important human complement regulators, FHL-1/reconectin and Factor H. All serum-resi stant B. afzelii isolates bound FHL-1/reconectin and also Factor H, and all analyzed serum-sensitive B. garinii isolates showed no or a significantly lower binding activity. Using recombinant deletion mutants, the binding dom ains were localized to the C terminus of FHL-1/reconectin to short consensu s repeats 5-7. The borrelial binding proteins were located in the surface o f the bacteria as demonstrated by immunofluorescence staining of intact, se rum-exposed bacteria and by enrichment of outer membrane proteins. The surf ace-attached complement regulators maintained complement regulatory activit y as demonstrated in a cofactor assay. By ligand blotting two different bor relial binding proteins were identified that were responsible for the surfa ce attachment of FHL-1/reconectin and Factor H. These borrelial complement regulators acquiring surface proteins (CRASP) were further characterized as either CRASP-1, a 27.5-kDa molecule which preferentially binds FHL-1/recon ectin and which was present in all serum-resistant borreliae, or CRASP-2, a 20/21-kDa protein which interacts preferentially with Factor H and the exp ression of which was more restricted, being detected in four of the six iso lates analyzed. In summary, we describe a new immune evasion mechanism of B . burgdorferi, as these bacteria acquire human complement regulators to con trol complement activation on their surface and to prevent formation of tox ic activation products.